Experimental studies of ulcerative colitis using the carrageenan animal model indicate that constituents of the normal intestinal microflora are necessary for initiation of experimentally induced ulcerations. It has been found that metronidazole, an antimicrobial agent active versus obligate anaerobes, suppresses ulceration. In addition, germfree animals do not develop large bowel ulcers when exposed to carrageenan, but re-conventionalized animals develop typical lesions. We propose to selectively simplify the bacterial flora necessary to cause cecal ulcerations using carrageenan as an inducing agent, in gnotobiotic guinea pigs by repopulating germfree animals with pools of bacterial isolated or individual species obtained from stool of animals with established ulcerative disease. Heat-killed bacteria, sonicated bacterial cultures and culture filtrates will be used to see whether viable bacteria, cell components or bacterial metabolites are required for ulceration. We have also noted a cytotoxic substance to be present in the stools of patients with ulcerative colitis and guinea pigs with carrageenan colitis. We will attempt to identify for microorganism(s) responsible for toxin production by screening strains isolated from these stools in a cytotoxicity tissue culture assay. The toxin-producing strains will then be tested in the germfree guinea pig fed carrageenan to assess their ability to promote colitis. Finally, gas-liquid chromatography will be employed to determine whether characteristic fatty acid profiles can be identified during ulcerative colitis. The intention of these studies is to identify specific bacterial species responsible for carrageenan induced ulcerative colitis and to define the mechanism of bacterial activity. The ultimate goal is to determine the etiology of ulcerative colitis in order to apply the lesson to future studies in man.